Friction stir welding tool and process for welding dissimilar materials

ABSTRACT

A friction stir welding tool and process for lap welding dissimilar materials are detailed. The invention includes a cutter scribe that penetrates and extrudes a first material of a lap weld stack to a preselected depth and further cuts a second material to provide a beneficial geometry defined by a plurality of mechanically interlocking features. The tool backfills the interlocking features generating a lap weld across the length of the interface between the dissimilar materials that enhances the shear strength of the lap weld.

This invention was made with Government support under ContractDE-AC0676RLO-1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to friction stir welding toolsand processes. More particularly, the invention is a friction stirwelding tool and process for lap welding dissimilar materials together.

BACKGROUND OF THE INVENTION

Friction stir welding (FSW) is a process for joining metals withoutfusion or filler materials. FSW is used routinely for joining componentsmade of aluminum and its various alloys. Indeed, it has beenconvincingly demonstrated that the process results in strong and ductilejoints, sometimes in systems which have proved difficult usingconventional welding techniques. The process is most suitable forcomponents which are flat and long (plates and sheets) but can beadapted for pipes, hollow sections and positional welding. The welds arecreated by the combined action of frictional heating and mechanicaldeformation due to a rotating tool. However, joining dissimilarmaterials with significantly different properties (e.g., meltingtemperatures and densities) is problematic for most welding methods,because the lower temperature melting material can liquefy and beremoved from the desired bonding area before the higher meltingtemperature material melts and before the weld can form. In general,conventional FSW between dissimilar materials yields unstable lap weldjoints due to the vastly different melt temperatures and flow stressproperties of the materials. Wide statistical deviation in the resultinglap welds is a common result.

The present invention disclosed herein provides for lap welding betweendissimilar materials. Additional advantages and novel features of thepresent invention will be set forth as follows and will be readilyapparent from the descriptions and demonstrations set forth herein.Accordingly, the following descriptions of the present invention shouldbe seen as illustrative of the invention and not as limiting in any way.

SUMMARY OF THE INVENTION

The invention is a friction stir welding tool and process for lapwelding dissimilar materials together. The tool includes a scribe cutterthat is integrated with, and radially positioned off center from, a pincomponent of the tool. The scribe cutter extends a preselected distancefrom the surface of the pin component. The scribe cutter is configuredto plunge through a first material positioned atop a second material ina lap weld configuration to a preselected depth that cuts a preselectedportion of the second material, which provides a geometry that includesa plurality of mechanically interlocking features in the surface of thesecond material component. The first material extruded by the toolbackfills the mechanically interlocking features that generates a lapweld across the length of the interface between the first and secondmaterials with enhanced shear strength. Shear strengths of the lap weldjoints can be in excess of 90% of the strength of the weaker material inthe lab weld stack. In one embodiment, the scribe cutter includestungsten carbide. In various embodiments, the scribe cutter includes acomponent selected from, but not limited to, e.g., nickel, titanium,tungsten, steel, carbide steel, polycrystalline cubic boron nitride,silicon nitride, rhenium, boron, and combinations of these materials.The scribe cutter extends a distance from the surface preferably in therange from about 0.1 mm to about 1.0 mm, but is not limited. The scribecutter includes a radial offset distance that is at least about onequarter of the diameter of the base of the pin component. The scribecutter is coupled to the pin component that can include a taper anglegreater than or equal to about 90 degrees. The pin component may includescroll threads or other features positioned along the length of the pincomponent that rotate in a clockwise or counter clockwise direction todrive first material extruded by the scribe cutter in the lap weld stackto the center line of the lap weld for incorporation therein. The scribecutter provides a rotational velocity of preferably between about 100rpm and 1000 rpm, but is not limited. The scribe cutter provides aplunge depth in the second material that is less than or equal to thelength of the scribe cutter. The scribe cutter generates a weldinterface with a width that is at least about two times the radialoffset distance of the scribe cutter. Other radial offset distances canbe selected in other embodiments. The scribe cutter contacts the first(or top) material as it plunges through the lap weld stack and cuts thesurface of the second (or lower) material in the lap weld stack betweenthe two materials forming mechanical interlocking features. Theinvention tool further includes a shoulder portion that surrounds thepin component at the base. The shoulder portion includes a surface thatmay be concave or convex. The shoulder portion may further have a smoothsurface or a featured surface that includes scroll grooves defined byconcentric spacings that deliver the first material extruded by thescribe cutter. The shoulder portion is positioned near the base of thepin component in relation to the plunge direction. The scribe cutterbackfills the mechanical interlocking features in the second materialforming the lap weld joint. The scribe cutter extrudes the first (ortop) material in the lap weld stack at below its melting temperaturesuch that the first material maintains a shear stress characteristic ofthe solid state, but that allows it to fill the mechanicallyinterlocking features introduced into the surface of the second materialforming the lap weld. The scribe cutter extrudes the first material suchthat the first material fills the mechanical interlocking features at asubstantially uniform hydrostatic pressure. The pressure selected is afunction of the material type and shape of the friction stir tool. Thescribe cutter maintains an operating temperature for the second materialthat is below the melting temperature of the first material. The scribecutter is angled with respect to the vertical direction at an anglebetween 0 degrees (i.e., that is aligned in the tool plunge direction orthe vertical direction) and 90 degrees (i.e., that is aligned at rightangles to the plunge direction, or the horizontal direction). Thecutting scribe can produce lap welds between dissimilar materials withincreased shear strength and a lower statistical deviation compared tolap welds produced absent the cutting scribe. The cutting scribe yieldslap welds with mechanical interlocking features that enhance the shearstrengths of the welds. Shear strengths between the selected dissimilarmaterials are a function of the types of materials used, melting points,densities, and hardness characteristics of the selected materials. Thescribe cutter of the lap weld tool provides a cutting depth in thesecond material that is less than or equal to the length of the scribecutter. In one embodiment, the scribe cutter cuts a preselected portionfrom the second material that defines a weld interface with a centerline for forming the lap weld. The weld interface includes a widthdefined by the radial offset dimension of the scribe cutter. The radialoffset distance of the scribe cutter can be varied. In a preferredembodiment, radial offset distance is at least about ¼^(th) of the pintip diameter off. In one embodiment, the diameter of the scribe cutteris about 0.031 inches (0.79 mm), but is not limited. The scribe cutterextends a preselected distance from the surface of the pin component. Inone embodiment, the scribe cutter extends to a height of about 0.070inches from the surface (face) of the pin component. The scribe cuttercan further include a positioning angle relative to the verticaldirection of less than about 90 degrees. In various embodiments,dissimilar materials in the lap weld stack can include: aluminum,magnesium, titanium, or alloys thereof; steel or steel alloys; ceramics;polymers; and combinations of these materials, described herein. Thefirst dissimilar material and the second dissimilar material have amelting temperature that is preferably different from the other by atleast about 20%. Alternatively, the first dissimilar material and thesecond dissimilar material have a density that is preferably differentfrom the other by at least about 10%.

The purpose of the foregoing abstract is to enable the United StatesPatent and Trademark Office and the public generally, especiallyscientists, engineers, and practitioners in the art who are not familiarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The abstract is neither intended to define theinvention of the application, which is measured by the claims, nor is itintended to be limiting as to the scope of the invention in any way.

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions the preferred embodiment of theinvention is shown and described by way of illustration of the best modecontemplated for carrying out the invention. As will be realized, theinvention is capable of modification in various respects withoutdeparting from the invention. Accordingly, drawings and descriptions ofthe preferred embodiment set forth hereafter are to be regarded asillustrative in nature, and not as restrictive.

A more complete appreciation of the invention will be readily obtainedby reference to the following description of the accompanying drawingsin which like numerals in different figures represent the samestructures or elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective side view of one embodiment of the invention.

FIG. 2 shows an enlarged side view of one embodiment of the invention.

FIG. 3 is a front face view of a preferred embodiment of the invention.

FIG. 4 is an enlarged side view of one embodiment of the invention.

FIGS. 5 a-5 f illustrate various geometries for the scribe cutter,according to various embodiments of the invention.

FIGS. 6 a-6 b illustrate plunge features of the scribe cutter of theinvention for forming lap welds between dissimilar metal components,according to a preferred embodiment of the invention.

FIG. 7 shows a typical process for forming a lap weld, according to anembodiment of the process of the invention.

FIG. 8 a-8 b compare lap welds produced by the invention and a prior artprocess.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a lap weld forming tool and process of a friction stirweld (FSW) design that generates lap welds between dissimilar materialswith enhanced joint strengths. As used herein, the term “dissimilar” inreference to lap weld component materials being joined means: “adifference in melting (temperature) point of more than about 20%, or adifference in density of at least about 10% by mass. The lap weldforming tool and process of the present invention overcome the chemicalincompatibility between dissimilar materials and components generating alap weld that binds the dissimilar materials (e.g., Mg to steel)together. The chemical incompatibility is overcome in two distinct ways.First, the lap weld produced by the invention chemically bonds thematerial components together using sufficient hydrostatic pressure andheat. Secondly, the lap weld forming tool includes a scribe cutterdetailed further herein that introduces features into the surface of thesecond material (i.e., the component having the higher meltingtemperature or higher density) (e.g., steel), which is placed generallyat the bottom of the lap weld stack along the length of the weldinterface. The term “lap weld stack” as used herein in reference tomaterials being joined refers to the arrangement in which at least afirst material is stacked atop at least a second material. A region ofoverlap is established between the dissimilar materials as a weldinterface between the components being joined together. The mechanicalinterlocking features introduced into the surface of the second materialcomponent are backfilled with the first material that is extruded by thescribe cutter and delivered by the lap weld forming tool. The filledinterlocking features enhance the shear strength of the lap weldsformed. As such, the invention provides lap weld joints betweendissimilar materials that appear to be bonded both chemically andmechanically. Lap welds of the invention thus exhibit lesssusceptibility to variations in sheet thickness and surface conditionsof the selected dissimilar materials.

The lap weld forming tool of the invention will be described herein inreference to formation of lap welds between two dissimilar materials,magnesium (Mg) as a first material component, and steel and steel alloysas a second material component. While tests will be described inconjunction with these exemplary materials, it is to be strictlyunderstood that the invention is not limited thereto. No limitations areintended.

The following description includes the preferred best mode of oneembodiment of the present invention. Basics for construction andoperation of the invention are also detailed hereafter. It will be clearfrom this description of the invention that the invention is not limitedto these illustrated embodiments but that the invention also includes avariety of modifications and embodiments thereto. Therefore the presentdescription should be seen as illustrative and not limiting. While theinvention is susceptible of various modifications and alternativeconstructions, it should be understood that there is no intention tolimit the invention to the specific form disclosed, but, on thecontrary, the invention covers all modifications, alternativeconstructions, and equivalents falling within the spirit and scope ofthe invention as defined in the claims.

FIG. 1 shows a lap weld forming tool 100 of a friction stir weld (FSW)design for joining dissimilar metal components, according to anembodiment of the invention. Lap weld tool 100 includes a body 2. In thefigure, a shank 4 couples centrally (i.e., in the middle of) to body 2,but is not limited thereto. Tool 100 includes a pin component 6 of atapered and threaded design that couples centrally on face 5 of shank 4that surrounds pin 6 at the base of pin 6. A scribe cutter 10 of apreselected, non-limiting length is integrated on face 8 of pin 6.Scribe cutter 10 is positioned a preselected distance radially offcenter on the face 8 of pin 6. In the exemplary embodiment, scribecutter 10 is composed of tool grade tungsten carbide. Tool body 2, shank4, and pin 6 are constructed of hardened H-13 tool steel, but componentmaterials are not limited thereto. As will be understood by the personor ordinary skill in the art, tool 100 may be of a unibody constructionor otherwise constructed of a single tool material. Thus, no materiallimitations are hereby intended. In the exemplary embodiment, scribecutter 10 is of a cylindrical design, but shape of scribe cutter 10 isnot limited to cylindrical shapes, as described further herein. Scribecutter 10 includes sharpened leading edges (not shown) that penetrateand cut through, materials (e.g., metals, ceramics, polymers, and otherselected materials described herein) being joined in a lap weld 25.Scribe cutter 10 penetrates through the first material 22 positionedatop second material 24 of the lap weld stack 25 and extrudes firstmaterial 22 to a preselected depth. Scribe cutter 10 then cuts a surfaceof the second material 24 in the stack 25 to form a geometry thatincludes a plurality of mechanically interlocking features (describedfurther in reference to FIG. 7 b). Scroll grooves 14 of pin component 6and scroll grooves 20 of shoulder 16 move first (or top) material 22extruded by scribe cutter 10 and backfills the extruded material intothe mechanically interlocking features along the length of the interface28 between the first 22 and second materials 24 that yields the lap weld28 between the dissimilar materials (22, 24). The presence of themechanically interlocking features enhances the shear strength of thelap weld 28, as described further herein. The off center radial positionof scribe cutter 10 on face 8 of pin 6 determines the width of or lengthacross lap weld 28 formed between the selected material components (22,24). In the exemplary embodiment, scribe 10, includes a height measuredfrom the surface 8 of pin component 6 of between about 0.1 mm and about0.5 mm, but is not intended to be limited thereto. For example, scribe10 can include a height defined as a percentage of the length of pincomponent 6 from about 1% to about 25% of the length of the pin. Thus,no limitations are intended to dimensions of the exemplary embodiment.In a preferred embodiment, lap weld forming tool 100 is of a convexscroll design described further herein in reference to FIG. 4. Inexemplary lap weld tests, lap weld tool 100 of the invention was testedby joining dissimilar metal materials together. In exemplary tests,magnesium (Mg) metal was joined as a first material sheet component 22together with various steel alloys as a second material component 24,described hereafter.

Lap Weld Forming Materials

Various combinations of dissimilar materials can be joined via lap weldin conjunction with the invention. Suitable materials include adifference in melting temperature of at least about 20%, a difference indensity of at least about 10%, and differences of at least about 10% inhardness and viscosity. Materials include, but are not limited to, e.g.,metals and metallic materials, polymers and polymeric materials,ceramics and ceramic materials, as well as combinations of thesematerials. Material combinations include, but are not limited to, e.g.,metal-metal combinations, polymer-polymer combinations, metal-polymercombinations, metal-ceramic combinations, polymer-ceramic combinations,and like material combinations. All dissimilar materials as will beselected by those of ordinary skill in the art in view of the disclosureare within the scope of the invention.

FIG. 2 shows an enlarged side view of lap weld tool 100. In the figure,scribe cutter 10 is positioned radially off-center on face 9 of pin 6.Pin 6 is of a tapered and threaded design that couples centrally toshank 4, defining a shoulder 16 that surrounds pin 6 at the base 12 ofpin 6. Shank 4 couples centrally to body 2. Pin 6 and shoulder 16include a series of scroll grooves 14 (e.g., ˜5 turns and 2.5 turns,respectively) that in the exemplary embodiment turn in a direction thatdrives material 22 extruded by scribe cutter 10 to the centerline (i.e.,placed at the center of) along interface 28 when tool 100 is rotated ata preselected rate or velocity, described further in reference to FIG.4. In the exemplary embodiment, shoulder 16 has a preferred,non-limiting diameter of about 12.5 mm. The material 22 extruded byscribe cutter 10 is placed into the mechanical interlocking features(described further in reference to FIG. 7 b) introduced by scribe cutter10 into second material 24 as scribe cutter 10 moves along the length ofinterface 28 between dissimilar materials (22, 24) being joined. Thismechanical interlocking geometry along the interface 28 between thedissimilar materials (22, 24) enhances the shear strength of lap weld 28that forms.

FIG. 3 shows a front view of face 8 of lap weld forming tool 100 of theinvention. In the figure, tool 100 includes a pin component 6 thatincludes a series of scroll grooves 14 that in the current configurationturn in a counter-clockwise (CCW) direction at a preselected rate,described further in reference to FIG. 4. Tool 100 further includes ashoulder 16 configured with a series (e.g., 2.5 turns) of scroll grooves20 that also turns in a counter-clockwise direction when tool 100 isrotated at a preselected rate. Number of grooves and turns is notlimited. Turn direction is also not limited. In the figure, scribecutter 10 is shown as an integrated component positioned radiallyoff-center on face 8 of pin 6. Scribe cutter 10 includes leading sharpcutting edges (not shown) that provide the cutting, penetrating, andplunging into various materials and components required to form the lapwelds between dissimilar materials. Shape of scribe cutter 10 and itscutting edges are not limited, as described further herein.

FIG. 4 shows an enlarged profile view of an exemplary embodiment of lapweld forming tool 100, including radial and structural dimensions. Inthe figure, tool 100 includes a pin component 6 that couples to shankcomponent 4 forming a shoulder 16. Shoulder 16 is of a convex tapereddesign that includes threaded scrolls 20 that drive material 22 extrudedby scribe cutter 10 to the centerline of the lap weld interface(described further in reference to FIG. 6 a). In the exemplaryembodiment, pin component 6 includes a diameter across face 9 of about0.17 inches (0.4 mm), but is not limited thereto. In one embodiment,scribe cutter 10 is integrated on the face 9 of pin 6 and extends fromthe surface a preselected height of 0.10 inches (0.254 mm). In anotherembodiment, scribe cutter 10 extends from the surface to a height ofabout 0.25 mm from the surface and has a width (diameter) of about 0.8mm. In the exemplary embodiment, radial distance of scribe cutter 10 onface 8 of pin 6 is preselected between about 1.0 mm and 1.8 mm from thecenter of pin 6 on face 8. As will be understood by the person orordinary skill in the art, distance that scribe cutter 10 extends fromthe surface of pin 6 can be varied so as to provide a variety ofpenetration (plunge) depths through various lap weld stacks 25 assembledwith dissimilar materials of various thicknesses. Thus, thickness ofmaterials is not intended to be limiting. Thicknesses of componentmaterials can be selected in the range from about 0.5 mm to about 50.0mm. Thus, no limitations are intended. In the exemplary embodiment, tool100 employs magnesium and steel component materials. Preferred thicknessis between about 2.1 mm and about 2.5 mm, but is not limited thereto. Inthe exemplary embodiment, pin 6 also includes a 10° taper angle, butangle is not limited thereto. The taper incline increases from the topof face 9 down the length of pin 6 to its base 12, where pin component 6couples to the shank component 4 forming shoulder portion 16. Pincomponent 6 includes scrolls (threads) 14 (e.g., 2 starts, ˜3.25 turns)that in operation rotate in a counter clockwise (CCW) direction.Direction is not limited. Shoulder 16 of the exemplary embodiment alsoincludes a series of scrolls 20 (e.g., 2 starts, ˜2.5 turns). In thefigure, each scroll 20 of shoulder 16 has an exemplary thread dimensionthat is 0.005 inches high and 0.01 inches wide, but dimensions are notintended to be limited thereto. The diameter of shoulder scrolls 20increases progressively from the interior edge of the shoulder 16diameter to the outermost edge of the shoulder 16 diameter. Scrolls (14,20) of the exemplary embodiment turn in a counter-clockwise directionwhen pin 6, shoulder 16, (and scribe cutter 10 of tool 100 rotate. Inoperation, pin component 6 of tool 100 turns (rotates) at a preselectedrate preferably in the range from about 100 rpm to about 1000 rpm, butrate is not a limiting parameter. Shoulder 16 of the exemplaryembodiment further includes a raised convex surface (˜1.60″ radius) 18that assists movement of extruded material that fills mechanicalinterlocking features (described further in reference to FIG. 7 b)formed in second material 24 by scribe cutter 10. The mechanicalinterlocking features ultimately enhance the strength of the lap weld(described in reference to FIG. 5 a) in lap weld stack 25 between thedissimilar materials (22, 24).

FIGS. 5 a-5 f show various alternate geometries for scribe cutter 10. InFIG. 5 a, scribe cutter 10 is of a substantially cylindrical design, asdescribed previously herein in reference to the exemplary embodiment,but shapes are not limited thereto. For example, in other embodiments,shape of scribe cutter 10 includes, but is not limited to, e.g.,rectangular (FIG. 5 b), triangular and pyramidal (FIG. 5 c), and conical(FIG. 5 d). In yet other embodiments, scribe cutter 10 is of astructured design that includes, but is not limited to, e.g., tapereddesign (FIG. 5 e), a threaded design (FIG. 5 f), and othernon-cylindrical geometries, including combinations of these variousdesigns. No limitations are intended.

Plunge Features

FIGS. 6 a-6 b illustrate the unique plunge features provided by thescribe cutter 10 of lap weld tool 100 of the invention for forming lapwelds between dissimilar materials, according to a preferred embodimentof the invention. The length of scribe cutter 10 of lap weld formingtool 100 allows the preselection of various plunge depths, given thatthe pin 6 and shoulder 16 components of tool 100 preferably do notcontact the second (bottom) material 24 in lap weld stack 25 duringformation of the lap weld—a unique property of the invention. In the lapweld forming process, lap weld forming tool 100 with its attached orintegrated scribe cutter 10 penetrates through the first (top) material22 (e.g., Mg) of lap weld stack 25 and plunges to a preselected depththat contacts and cuts the surface of second material 24 (e.g., steel),but avoids contact with the pin component 6 or shoulder 16. Thisconfiguration ensures tool 100 will produce insufficient heat to meltthe first, or lower melting, material component 22, yet allows scribecutter 10 to penetrate through, extrude, and mix the first materialcomponent 22. The preselected plunge depth reached by scribe cutter 10through lap weld stack 25 provides contact with, and cuts a beneficialgeometry on, a preselected portion of the surface of second material 24.This is a fundamentally different approach than is undertaken with FSWtools and processes known in the prior art. In particular, scribe cutter10 of the present invention introduces a geometry that forms mechanicalinterlocking features (FIG. 8 a) into the surface of second material 24between dissimilar materials (22, 24) along the length of the interface28 that defines lap weld 28. These mechanical interlocking features arebackfilled with first material 22 that is extruded by scribe cutter 10as it plunges and moves through the lap weld stack 25 into secondmaterial component 24 along weld interface 28. Position of scribe cutter10 on the pin component 6 allows the width or area across the interface28 to be varied or preselected without increasing the size of tool 100.Lap weld forming tool 100 of the invention further minimizes heatrequired to form lap weld 28, which minimizes deleterious effectsassociated with excessive heat. Presence of mechanical interlockingfeatures (FIG. 8 b) further enhances the shear strength of lap weld 28,while simultaneously minimizing statistical deviation in joint strengthsassociated with formation of the lap weld, and providing reproduciblelap welds in accordance with the invention as described further herein.Mechanical interlocking is made possible by differences in the meltingtemperatures, densities, and other associated properties between thedissimilar materials (22, 24) selected. Such differences and extremes inmaterial properties are not experienced by prior art FSW devices andprocesses because the materials to be joined are largely similarproperties. Thus, the invention provides a lap weld (joint) 28 betweenselected dissimilar materials (22, 24) that is appears to be bothchemically bonded and mechanically bonded. Pressures required by thepresent invention to penetrate component materials (22, 24) in lap weldstack 25 are not intended to be limited. For example, pressures willdepend on the materials being joined, the hardness of selectedmaterials, the thickness of materials being joined, the rate of rotationof the scribe cutter 10, and other welding parameters including, but notlimited to, e.g., plunge velocity, tool shape, plunge depth, and toolmaterials. Thus, no limitations are intended. The process for joiningdissimilar materials in conjunction with the invention will now bedescribed.

Solid State Joining of Dissimilar Materials

FIG. 7 shows a typical process 700 for joining dissimilar materials inaccordance with the invention. {START}. In an optional first step {Step702}, materials to be joined are cleaned, e.g., using isopropyl alcoholor another cleaning solution prior to welding. Next {Step 704},materials to be lap welded are arranged in a suitable lap weld stack 25or configuration. For the exemplary lap weld described herein, a sheetof magnesium (Mg) 22 of a preselected thickness (e.g., 2.3-mm to 2.5-mm)was placed atop a sheet of steel 24 of a similar thickness. Thicknessesof the dissimilar materials are not limited. Overlap width of first(top) component material 22 and second (bottom) component material 24 inthe exemplary lap weld stack 25 that defined lap weld interface 28 weretypically about 35-mm, but is not limited thereto. Next {Step 706}, lapweld forming tool 100 is positioned over the lap weld interface 28(centerline) of the overlapping material components (22, 24) and scribecutter 10 cuts second (bottom) material 24, introducing mechanicallyinterlocking features (FIG. 8 b) of a preselected depth into the surfaceof second material 24. Typical cut depth in surface of component 24 isabout 0.05″, but is not limited. For example, tool 100 can be plunged toa limit of about 95% of the thickness of the first (top) sheet 22 or upto the length of scribe cutter 10 such that the scribe 10 interfaceswith second material 24 without generating excessive heat that can meltthe lower melting material 22. Scribe cutter 10 has an exemplary lengthof about 0.010″, but is not limited thereto. Thus, in the exemplaryembodiment, plunge depth through material components (22, 24) of lapweld stack 25 is preselected in the range between about 0.003″ and about0.007″ depending on the thickness of the second material 24, but is notlimited thereto. Following penetration into lap weld stack 25, lap weldforming tool 100 proceeds, e.g., in the X-dimension, placing material 22extruded by scribe cutter 10 along the centerline of weld interface 28as tool 100 rotates and scrolls (e.g., in the counter-clockwisedirection). A sufficient pressure and heat (that are functions of bothtool geometry and process parameters) are selected to extrude materialfrom the first material component 22 that serves to move this materialinto the interlocking features (FIG. 8 b) introduced into secondmaterial 24 along the interface 28 between dissimilar materials (22,24). The invention further enables the FSW process in that no melting oftop sheet 22 occurs between dissimilar materials (22, 24). Lap weldforming tool 100 in combination with scribe cutter 10 generatessufficient forging loads and thermal heat to reduce yield and flowstresses of first (top) material 22 without melting it. Next {Step 708},scribe cutter 10 backfills the mechanical interlocking features (FIG. 8b) introduced into the surface of second material 24 with the firstmaterial 22 extruded by scribe cutter 10, which enhances the shearstrength of the lap weld 28 formed between dissimilar metal components(22, 24) along the length of interface 28 {END}.

Microstructure of the Lap Weld

FIGS. 8 a-8 b are cross-sectional views of lap weld joints produced by aconventional Friction Stir Weld (FSW) process (FIG. 8 a) and theinvention (FIG. 8 b), respectively, that compare the microstructure ofthe welds. In FIG. 8 a, the conventional FSW lap weld joint 28 shows asmall void 26. The convention joint exhibits a low shear stresstolerance. In FIG. 8 b, in contrast, lap weld joint 28 of the inventionincludes mechanical interlocking features 30 along the length of the lapweld interface 28. These mechanical interlocking features 30 areintroduced into the surface of second material 24, as scribe cutter(FIG. 4) of lap weld forming tool 100 advances horizontally from rightto left into the photograph plane. The mechanical interlocking features30 are backfilled with first material 22 (e.g., Mg) extruded by scribecutter (FIG. 4) as it advances along the length of the lap weldinterface 28 through lap weld stack 25. The mechanical interlockingfeatures 30 provide one binding mechanism that secures the dissimilarmaterials (22, 24) in lap weld 28 together, which serves to enhance theshear strength of the lap weld 28. In exemplary lap welds of theinvention produced between dissimilar materials composed of magnesiumand various steels and steel alloys, the lap welds 28 exhibited shearstress yields greater than about 90% of the strength of the individualmaterials (22, 24) forming the lap weld 28. Typical load stresses andtemperatures employed by the invention depend on the tool design,rotation, transverse translation speeds, applied pressure, as well asthe material properties between the dissimilar materials being joined.The central plunge region that produces lap weld 28 contains acharacteristic “onion-ring” flow pattern, which is the most severelydeformed region of lap weld 28. The layered onion-ring structure is aconsequence of the way in which scroll grooves (14, 20) of the tool 100deposit material 22 extruded by scribe cutter 10 from the front to theback of the weld 28 as the cutter 10 rotates in the interface 28 betweenthe materials (22, 24) being joined. Designs of the lap weld formingtool 100 of the present invention concentrate on the ratio between thepin 6 and the shoulder 16; preferred diameters are in a ratio of about1:3. Rotational aspects of scribe cutter 10, scrolls (14, 20), pincomponent 6, and shoulder 16 are designed to influence the overall flowof first (top) material 22 into the mechanical interlocking features(FIG. 8 b). For example, when joining materials with greatly differingflow stresses and melting regimes, the invention tool 100 does not mixthe two materials. Conventional understanding of linear friction stirwelding of lap joints prior to the invention was that a FSW tool shouldpenetrate (plunge) entirely through the material of upper sheet 22.However, experiments with conventional linear friction stir weldingdevices demonstrated that plunging a FSW tool into the lower sheet 24 ofa lap weld stack 25 configured with materials with melting points thatdiffered by at least 20% quickly generates temperatures that melt thefirst (top and less dense) material, forming unstable lap weld jointswith insufficient load and shear stress strengths. Tests havedemonstrated, for example, that contact between a pin component and ahigh temperature melting material (e.g., steel) produces excessive heatthat proves to be problematic to the formation of a proper lap weldjoint between dissimilar materials (22, 24). For example, in theexemplary embodiment described herein, attempts to join a Mg sheet 22 (arelatively low melting temperature metal) to steel sheets 24 and othersteel alloys (significantly higher melting temperature metals) using aconventional FSW tool caused excessive flash, problematicmicrostructures, and other related bonding problems. In the presentinvention, introduction of scribe cutter 10 of lap weld forming tool 100described herein prevents overheating of the lower melting temperaturematerial 22. Scribe cutter 10 provides an effective geometry and area ofcontact on the high melting temperature metal component 24 for bondingthe dissimilar materials (22, 24) together. Further, temperatures areselected such that they do not exceed 80% of the melting point of thelower melting metal component material 22. In addition, externalpressures used for the lap welding process are generally not critical.Selected pressures are a function of the tool design, materials beinglap welded, and plunge depths employed.

The exemplary embodiment of the invention was tested by measuring shearstress strength of lap welds between sheets of magnesium and steelalloys. The lap weld forming tool 100 integrates a small scribe cutter(i.e., an integral scribe) at the bottom of a pin component of afriction stir weld tool that includes a hardened or abrasive surface. Inoperation, the integral scribe cutter 10 of tool 100 produces a verysmall area of penetration through the first material 22 through lap weldstack 25 through the interface 28 defined between dissimilar materials(22, 24) and into second material 24. Only the scribe 10 of theinvention contacts the surface of the 2^(nd) component material 24,thereby eliminating the excessive heat associated with conventional FSWtools and processes. The scribe cutter 10 then disrupts and cuts thesurface of second material component 24 introducing mechanicalinterlocking features (FIG. 8 b) into the second material component 24that enhances the strength of the lap weld 28 formed between dissimilarmaterials (22, 24). A suitable temperature and pressure of the FSW stirweld turning process allows the softer material 22 component (e.g., Mg)in the dissimilar lap material stack 25 to fill the mechanicalinterlocking features (FIG. 8 b) produced in the harder second materialcomponent 24.

The following examples will provide a further understanding of theinvention in its larger aspects.

Example 1 Statistical Deviation Of Invention Lap Welds

A lap weld (250-mm line length) produced by the invention between a2.3-mm thick sheet of a magnesium alloy (e.g., AZ31 alloy) and a 0.8-mmthick sheet of U.S. Steel Drawing Type B-Hot Dipped Galvanizing(DSTB-HDG) steel(http://www.uss.com/corp/auto/tech/grades/lowcarbon/ds_type_b.asp) gavea shear strength of 210.4 kN/m with a deviation of +/−5.06 (83% to 87%of the tensile strength for the 0.8 mm steel). A lap weld produced foridentical materials using a conventional FSW tool without the scribecutter demonstrated a shear strength of 188.4 kN/m with a deviation of+/−60.5 (37% to 84% of the tensile strength of the 0.8-mm steel).Results show an increase in the strength of the invention lap weld of atleast about 25% on average compared to the conventional weld.Furthermore, statistical deviation of the lap weld shear strength wasreduced from 60.5 kN/m to 5.06 kN/m.

Example 2 Load Tolerance of Lap Weld #1

A lap weld made in conjunction with the invention between a 2.3-mm thicksheet of a magnesium alloy (e.g., AZ31) and a 0.8-mm thick sheet of U.S.Steel DSTB-HDG steel alloy demonstrated a load tolerance of ˜6500N (245kN/m). Normal load tolerances for AZ31 (2.3-mm) and DSTB-HDG (0.8-mm)are ˜624 kN/m and ˜247 kN/m, respectively. Results show the loadcapacity for invention lap welded materials to be at or near the bearingcapacity of the weaker material (DTSB-HDG).

Example 3 Load Tolerance of Lap Weld #2

Another lap weld made in conjunction with the invention combined a2.3-mm thick sheet of magnesium alloy (e.g., AZ31) and a 1.5-mm thicksheet of High Strength, Low Alloy Hot Dipped Galvanizing (HSLA-HDG)steel. The lap weld demonstrated a maximum load of ˜7600N (249 kN/m).Normal load tolerances for AZ31 (2.3-mm) and HSLA-HDG (1.5-mm) are ˜624kN/m and ˜896 kN/m, respectively. Results show the load capacity for theinvention lap welded materials to be at least about 40% of the bearingcapacity of the AZ31, a significant increase (greater than 20%) overstrengths of lap welds produced without the scribe cutter of theinvention.

Example 4 Load Tolerance of Lap Weld #3

Another lap weld made in conjunction with the invention combined a2.3-mm thick sheet of magnesium alloy (e.g., AZ31) to a 0.8-mm thicksheet of DSTB-HDG steel. The lap weld demonstrated a maximum load of˜6200N (214 N/m). Normal load tolerances for AZ31 (2.3-mm) and DSTB-HDG(1.5-mm) are ˜624 kN/m and ˜247 kN/m, respectively.

CONCLUSIONS

A new lap weld forming tool and scribe cutter have been described thatenable friction stir welding of dissimilar materials. The inventionprovides a wide variety of process parameters including, but not limitedto, e.g., material melting temperatures, material density differences,material hardness properties, material thicknesses, greater control overheat inputs, tool rotation rates (RPM), linear weld velocity, and likeprocess parameters. The invention scribe further provides the ability totailor the microstructure of the materials in a lap weld stack thatenhances the strength of the weld between the dissimilar materials. Inparticular, the invention adds a mechanical interlocking geometry intothe weld interface that increases the strengths of the lap welds andminimizes the deviation and scatter therein. Exemplary lap welds havedemonstrated shear strengths in excess of 90% of the base strength ofthe material components not previously known in the art.

While preferred embodiments of the present invention have been shown anddescribed, it will be apparent to those of ordinary skill in the artthat many changes and modifications may be made without departing fromthe invention in its true scope and broader aspects. The appended claimsare therefore intended to cover all such changes and modifications asfall within the spirit and scope of the invention.

1. A friction stir welding tool, characterized by: a scribe cutter thatextends from a radially offset position on a terminal end surface of apin, the pin is operatively positioned between a shank and the scribecutter having a preselected height that prevents contact by a piece ofmaterial to the shoulder of the shank when the scribe cutter cuts thematerial.
 2. The friction stir welding tool of claim 1, wherein thescribe cutter comprises tungsten carbide.
 3. The friction stir weldingtool of claim 1, wherein the scribe cutter includes a member selectedfrom the group consisting of: nickel, titanium, tungsten, steel, carbidesteel, polycrystalline cubic boron nitride, silicon nitride, rhenium,boron, and combinations thereof.
 4. The friction stir welding tool ofclaim 1, wherein the scribe cutter extends a distance from the surfaceof the pin selected in the range from about 0.1 mm to about 1.0 mm. 5.The friction stir welding tool of claim 1, wherein the scribe cutterincludes a radial offset distance measured from the center of thesurface that is at least about one quarter of the diameter of the baseof the pin.
 6. The friction stir welding tool of claim 1, wherein thescribe cutter includes a rotational velocity of between about 100 rpmand 1000 rpm.
 7. The friction stir welding tool of claim 1, wherein thescribe cutter is coupled to the pin that includes a taper angle greaterthan or equal to about 90 degrees.
 8. The friction stir welding tool ofclaim 1, wherein the scribe cutter provides a plunge depth in the secondmaterial that is less than or equal to the length of the scribe cutter.9. The friction stir welding tool of claim 1, wherein the scribe cuttergenerates a weld interface with a width that is at least about two timesthe radial offset distance of the scribe cutter.
 10. The friction stirwelding tool of claim 1, wherein the scribe cutter provides the firstmaterial extruded by same such that it backfills the mechanicalinterlocking features in the second material along the length of theweld interface forming the lap weld joint.
 11. The friction stir weldingtool of claim 1, wherein the scribe cutter extrudes the first materialat below the melting temperature thereof such that the first materialmaintains a local shear stress characteristic of the solid state thatallows it to fill the mechanical interlocking features in the secondmaterial forming the lap weld joint.
 12. The friction stir welding toolof claim 1, wherein the scribe cutter extrudes the first material suchthat the extruded first material fills the mechanical interlockingfeatures at a substantially uniform hydrostatic pressure.
 13. Thefriction stir welding tool of claim 1, wherein the scribe cutter isangled at between 0 and 90 degrees with respect to the verticaldirection.
 14. The friction stir welding tool of claim 1, wherein thescribe cutter maintains an operating temperature for the second materialthat is below the melting temperature of the first material.
 15. Thefriction stir welding tool of claim 1, wherein the cutting scribe yieldsa lap weld with increased shear strength and lower statistical deviationcompared to a lap weld generated absent the cutting scribe.
 16. A methodfor forming a lap weld between two dissimilar materials, the methodcomprising the steps of: stacking a first material that is dissimilarfrom a second material atop the other in a lap weld stack with anoverlap therebetween sufficient to form a weld interface of apreselected width along the length between the materials being joined;penetrating through the first material of the lap weld stack with ascribe cutter that extends from a radially offset position on a terminalend surface of said pin, extruding same to a preselected depth andcutting a surface of the second material to form a plurality ofmechanically interlocking features therein; and backfilling themechanically interlocking features along the length of the interfacewith extruded first material to form a lap weld between the twodissimilar materials with an enhanced shear strength.
 17. The method ofclaim 16, wherein the stacking step includes a first material that is ametal selected from aluminum, magnesium, titanium, or alloys thereof.18. The method of claim 16, wherein the second material is steel or asteel alloy.
 19. The method of claim 16, wherein the first dissimilarmaterial is a metal selected from: aluminum, magnesium, titanium, or analloy thereof; and the second dissimilar material is steel or a steelalloy.
 20. The method of claim 16, wherein the first dissimilar materialis a ceramic and the second dissimilar material is steel or a steelalloy.
 21. The method of claim 16, wherein the first dissimilar materialand second dissimilar material have a melting temperature that isdifferent from the other by at least about 20%.
 22. The method of claim16, wherein the first dissimilar material and second dissimilar materialhave a density that is different from the other by at least about 10%.23. The method of claim 16, wherein the penetrating step introduces cutsin the second material that are cross-sectional cuts.
 24. The method ofclaim 16, wherein the penetrating step includes extruding the firstmaterial at a temperature below its melting temperature.
 25. The methodof claim 16, wherein the lap weld has a shear strength that is at leastabout 80% of the strength of the lower melting material therein.